Investigation of Dracocephalum extract based on bulk and nanometer size as green corrosion inhibitor for mild steel in different corrosive media

In recent years, green corrosion inhibitors derived from natural plant resources have garnered much interest. In the present work, at first, we investigated the corrosion behavior of mild steel (st-37) in the presence, and absence of Dracocephalum extract based on bulk size as a corrosion inhibitor in two widely used acidic environments (0.5 M H2SO4, and 1.0 M HCl), at room temperature. Then, we used Dracocephalum extract based on nanometer size to reduce the optimal concentration of inhibitor, increase the corrosion resistant, and efficiency. Dracocephalum extract does not contain heavy metals or other toxic compounds, and also good characteristics such as low cost, eco-friendly, and widespread availability, make it suitable nature candidate as an environmentally safe green inhibitor. The anticorrosive behavior was assessed using electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PP). In all of the studies, the inhibitory efficiency (IE%) increased as the extract dose was increased. But by using nano extract, in addition to maintaining high efficiency, the amount of inhibitor was reduced significantly. The highest IE% is 94% at the best dose of nano extract (75 ppm), but the highest IE% is 89% at the best dose of the bulk extract (200 ppm) in H2SO4 solution. Also, for the HCl solution, the highest IE% is 88% at the best dose of nano extract (100 ppm), but the highest IE% is 90% at the best dose of the bulk extract (400 ppm), by polarization method. The PP results suggest that this compound has an effect on both anodic, and cathodic processes, and that it adsorbs on mild steel surface according to the Langmuir adsorption isotherm. Optical microscopy, scanning electron microscopy (SEM) analysis, and a solid UV–Visible reflection spectrum were used to investigate the alloys' surface morphology.

www.nature.com/scientificreports/ Distilled water was used to clean the substrates, which were then degreased with ethyl alcohol and dried at room temperature.
Preparation of Dracocephalum extract. The healthy leaves of Dracocephalum were purchased from the local markets in Iran, which are completely designated for commercial usage. To eliminate dust, the gathered leaves were gently washed. The leaves were dried in the shade at room temperature. At ambient temperature and in the dark, 100 g of dried Dracocephalum leaves were soaked in methanol for 72 h. The surplus solvent was evaporated under reduced pressure in a rotary evaporator at 40 °C after filtering the solution. The recovered residue had a consistent weight of 2.0 g. It is noteworthy that, alcohol-based herbal preparations are those that use some form of alcohol as the solvent. Herbal tinctures, and herbal liniments are both considered alcohol-based preparations even though two different types of alcohol are used (ethyl alcohol and isopropyl alcohol, respectively). Alcohol preparations have a long shelf life as alcohol slows the decomposition of materials and bacterial growth, thus, increasing herbal preparation shelf life 38 . Declaration for the usage of plant materials. We declare that in this research, we did not use or not going to use any plants (either cultivated or wild) irrespective of any location. Experimental research and field study in this study has complied with the IUCN Policy Statement on Research Involving Species at Risk of Extinction. The use of plants in the present study complies with international, national and/or institutional guidelines.
Preparation of Nanosized Dracocephalum extract. To obtain herbal nanostructures, the following method used. A specific value of pure Dracocephalum extract dissolved in 100 mL of ethanol in a beaker to have a solution. The solution stirred at room temperature via vigorous stirring for 30 min at 800 rpm, then the product filtered using filter papers (Whatman, 40 Ashless, Germany) to remove probable impurities. The filtered solution added at a 1:10 ratio to distilled water to isolate pure herbal particles. The suspensions placed in an ultrasonic bath for 20-30 min, and afterwards, to produce lower-sized nanostructures, ultra-prob sonication for 20 periods of 10 s (Hielscher, UP100H, Germany) used as well. Afterward, nanoparticles acquired in the colloid state. In this colloid, nanoparticles observed using dynamic light scattering (DLS) techniques.
Preparation of solutions. The corrosive media were 0.5 M H 2 SO 4 , and 1.0 M HCl, made by diluting analytical grade Merck H 2 SO 4 , and HCl with double distilled water, respectively. Before to each experiment, the test solutions were made fresh by mixing the extract with the corrosive solution. Experiments were conducted twice to verify repeatability. The extract concentrations were 50, 100, 150, 200, and 250 ppm for 0.5 M H 2 SO 4 , and 100, 200, 300, 400, and 500 ppm for 1.0 M HCl based on bulk size, and 25, 50, 75, and 100 ppm for 0.5 M H 2 SO 4 , and 50, 75, 100, and 125 ppm for 1.0 M HCl based on nanosized extract.
Noticeably, pentacyclic triterpenoids are one of the main functional components in Dracocephalum extract. Pentacyclic triterpenoids are practically insoluble in water and low concentration ethanol, but they are soluble in chloroform, HCl, and acidic media 39 . Characterization. To investigate the size distribution or average sizes of the plant extract, dynamic light scattering (DLS) was employed. DLS data obtained using a Nano-ZS90 (Malvern) apparatus (Malvern Instruments, Malvern, UK). Electrochemical research such as electrochemical impedance spectroscopy, and potentiodynamic polarization were done using the AutoLab device (302 N potentiostat, Netherlands). Scanning electron microscopy (SEM FEI Quanta 200, accelerating voltage 20.0 kV), and optical microscopy (Leica zoom 2000 model) were used to investigate the surface morphology of mild steel submerged in sulfuric acid, and hydrochloric acid without and with the optimal concentration of Dracocephalum extract. The measurements of UV-Visible reflection spectra of surface species on the mild steel were performed by using UV-Vis spectrophotometer A SPECORD 210 (Analytik Jena, Germany) in the stainless tank (π × 1 2 × 1.5 cm) to avoid interference from ambient light. This spectrophotometer is controlled with the Spectra Manager software. For the last two tests, working electrodes were mechanically polished, and immersed in 0.5 M H 2 SO 4 , and 1.0 M HCl solutions in the absence, and presence of an inhibitor for about 24 h at room temperature and then removed and dried.
Statistical analysis. After exploring the normal distribution using the Kolmogorov-smearnov test, the data were subjected to One Way ANOVA, and Tukey Post Hoc tests (S = 0.05).
Stability study. The synthesized nanoparticles were stored at 4 °C, room temperature (24 °C), and physiologic temperature (37 °C) for 3 weeks in the glass vials. After a duration of storage, the distribution of nanoparticles size considered to detect the variations in the formulation with respect to time.

Procedures
Electrochemical measurements. EIS is a vital way of monitoring in situ electrochemical changes with critical knowledge of physical processes occurring at the metal/electrolyte interface 40 , so impedance diagrams may provide information on mechanistic, surface characteristics, and electrode kinetics 41 . In most applications, the basic lab setup comprises employing three electrodes in the electrochemical cell for the measurement: working, counter, and reference electrodes submerged in a specified volume and the concentration test solution. So in this work, a three-electrode cell containing Pt electrode, Ag/AgCl electrode, and st-37 specimen as a counter, a reference, and a working electrode, respectively, have been used. First, the open circuit potential (OCP) was recorded for 30 min, and then the EIS data were obtained. The experiment is carried out using a modest potential of 10 mV of AC voltage and frequencies ranging from 100 kHz to 100 mHz. The inhibition efficiency (IE I ) of a corrosion inhibitor was estimated using the following equation utilizing electrochemical data collected from the workstation 42 : where R ct , and R′ ct are the polarization resistance of the sample in the presence, and absence of the corrosion inhibitor, respectively. Potentiodynamic polarization is another electrochemical-based method for determining the corrosion mechanism protection, corrosion rate, and effectiveness of green corrosion inhibitors. The experiment is carried out in a three-electrode electrochemical cell, the same as EIS. The polarization scan rate was set at 1 mV/s to plot the Tafel polarization curves. The electrode potential was changed automatically from − 800 mV to − 100 mV vs. E corr at 25 ± 1 °C to create these graphs. After EIS, a potentiodynamic test was used to determine the polarization curve. The corrosion inhibitor's inhibition efficiency (IE P ) is calculated using the following equation 43 : where, i, and i′ are the current densities of the solution in the absence, and presence of the inhibitor, respectively.
Also, using NOVA 1-10 software, the suitable equivalent circuit, the corresponding EIS, and potentiodynamic polarization parameters can be prepared.
To check the reproducibility of the results, at least two experiments were performed at each concentration for EIS, and potentiodynamic polarization curve. The standard deviations (S.D.) were obtained and, S.D. values were small, suggesting that the electrochemical measurements had good reproducibility. In this work, S.D. is smaller than 0.5 for all electrochemical experiments, so these data were omitted in the following sections.

Results and discussion
Characterization of the nanosized plant extract. Dynamic light scattering is used for measuring average particle diameter, and particle diameter distribution of nanosized particles dispersed in the liquid. Extract Biomolecules like proteins, enzymes, terpenoids, and flavonoids cofactors play both capping, and reducing role. Furthermore, due to strong binding ability with amino acid residues (carbonyl group), agglomeration behavior was prevented, and stability of medium was provided. For a better understanding of the real size of nanoparticles, the nanosizer technique used for calculating particle size, and stated by SBL (Statistical Bin Limits) analysis. For this propose, reduction of agglomeration fault to state actual particle size was done by omitting hydrodynamic radius. Figure 1 reported the histogram of the SBL nanosizer of NPs, which showed the mean diameter of the size of the particle is ~ 64.75 nm for nanostructures. Reported results demonstrated narrow size distribution, and homogenous dispersity of NPs.   44 . The general semi-circle shape of the curves is fairly constant across the whole inhibitor concentration range, indicating that no change in corrosion mechanism has occurred as a result of the addition of plant extract 45 .

Corrosion inhibition study.
High-frequency capacitance circuits are generally generated by charge transfer resistance, as demonstrated in Figs. 3a, and 4a. It can be seen that adding inhibitor causes an increase in the radius of the capacitive ring, and inhibits electrochemical processes to some extent. It appears that adding the extract to mild steel reduces  Tables 2, and 3 show the EIS characteristics for mild steel with various concentrations of Dracocephalum extract (bulk, and nanosize) in acidic media, including the goodness of fit (chi-square), solution resistance (R S ), double layer capacitance (C dl ), charge transfer resistance (R ct ), and the degree of surface coverage (θ = IE I /100). These semi-circles also show that IE I % increases with an increase in inhibitor concentrations. It is noted that the extract with nanosize possess better IE I % than the bulk extract, in the same amount, in both solutions.  www.nature.com/scientificreports/ Also, R ct increases when the concentration of Dracocephalum increases, owing to enhanced extract coverage on the steel surface, and higher inhibitor shielding efficiency against ion penetration of the corrosive medium 46 . When the inhibitor concentration is up to 200 ppm, and 75 ppm for 0.5 M H 2 SO 4 , and up to 400 ppm, and 100 ppm for 1.0 M HCl containing bulk, and nanosize of the extract, respectively, the R ct , and IE I % reaches the highest value (90, 92, 91, and 88%). This rise shows that the inhibitor builds an adsorption layer on the mild steel alloy's surface, preventing corrosion. R ct begins to decrease as the concentration of Dracocephalum extract increases, as the inhibitor is desorbed from the metallic surface. As the extract concentration grew, the electric double-layer capacitor, C dl , dropped, which may be ascribed to a decrease in the local electric double layer constant 47 . In this case, inhibitor molecules adhered to the steel surface, and replaced the original water molecules that were present in the steel surface's interface layer. The C dl decreased as the inhibitor concentration grew because the inhibitor molecules had a lower dielectric constant than water molecules, causing the inhibitor molecules to be loosely organized in the interface layer 48 . It was discovered that the extract might produce an inhibitor coating on the steel surface to prevent corrosion, indicating that Dracocephalum extract has high inhibition efficiency for mild steel.
The increase in phase angle with increasing extract content, as seen in the Bode plots in Figs. 3c,d, and 4c,d, further supports the prevention of corrosion 13 . The roughness of the electrode surface is linked to the value of the phase angle in these figures. The higher the value of θ, lower is the surface roughness. As the inhibitor concentration increases, the surface roughness decreases, implying that corrosion decreases.
The equivalent Randle's circuit model (Fig. 5) was used to examine all of the impedance curves illustrated in Figs. 3, and 4. This is made up of a series solution resistance (R S ), a parallel resistance (R ct ), and capacitor combination (C dl ).
Potentiodynamic polarization in H 2 SO 4 and HCl media. Figure 6 depicts the cathodic, and anodic polarization curves of mild steel following immersion in 0.5 M H 2 SO 4 , and 1.0 M HCl solutions in the absence, and presence of various amounts of extract. The experimental results including the corrosion current density (i corr ), the cathodic, and anodic Tafel slopes (β c , and β a ), the corrosion potential (E corr ), the inhibition efficiency (IE p %), and the degree of surface coverage (θ) for different solutions are reported in Table 4. The corrosion current density www.nature.com/scientificreports/ was calculated using the intercept of extrapolated cathodic, and anodic Tafel lines at the corrosion potential. Also, the IE p % was calculated using Eq. (2). From the experimental values, it can be observed that the corrosion current density decreases significantly with an increase in inhibitor concentration up to 200 ppm, and 75 ppm for 0.5 M H 2 SO 4 , and up to 400 ppm, and 100 ppm for 1.0 M HCl containing bulk, and nano size of the extract, respectively, supports the retardation of the corrosion process 49 . The reduced current density in the presence of inhibitor in all four solutions suggests that the metal surface is passivated due to the creation of the inhibitor layer 50 . The results reveal that i corr of mild  www.nature.com/scientificreports/ steel decreased from 1427 μA/cm to 151 μA/cm, and 1427 μA/cm to 91 μA/cm, and the IE% increased to 89%, and 94%, and also, 752 μA/cm to 73 μA/cm, and 752 μA/cm to 87 μA/cm, and the IE% increased to 90%, and 88%, for H 2 SO 4 , and HCl solutions with bulk, and nano size of the extract, respectively. The findings of the investigation, suggest that the nano extract of the plant has greater inhibitory properties than the regular extract. Furthermore, differences in the values of β c , and β a compared to blank solutions show that these inhibitors safeguard the corrosion process by adsorbing inhibitor molecules on both anodic, and cathodic sites.
With the addition of inhibitors, there is a distinct change in the cathodic, and anodic parts of curves in Tafel plots of H 2 SO 4 solution. As a result, it's referred to as a mixed-type inhibitor. From Fig. 6c,d, and Table 4, in hydrochloric acid solution, the shape of the anodic, and cathodic curves, and the Tafel parameter (β c , and β a ) did not change significantly after using the extract as an inhibitor, but in sulfuric acid solution β a changed (Fig. 6a,b), and this means that the inhibitor acts as a both anodic, and cathodic inhibitor (mixed one), with predominant anodic effect in H 2 SO 4 medium. On the other hand, for H 2 SO 4 , and HCl solutions, the maximum shift in E corr value is positive/negative side 41, and 15 mV, respectively, and a literature survey revealed that if a shift in corrosion potential is less than ± 85 mV with respect to the blank solution, the inhibitor acts as a mixed-type inhibitor; thus, this inhibitor is a mixed-type inhibitor 51 .
Based on the above analysis, the values of IE I %, and IE P % rise as the concentration of inhibitors rises, with bulk, and nano size of extract in acidic media. The mean difference between the maximum values of %IE I , and www.nature.com/scientificreports/ %IE P using the best concentration of extract with bulk and nano size is 1.0, and 2.0%, in H 2 SO 4 , and 1.0, and 0.0%, in HCl solutions, respectively. Figure 7 shows the influence of inhibitor concentration (ppm) on inhibition efficiency (IEp, and IE I , %) for st-37 steel in 0.5 M H 2 SO 4 , and 1.0 M HCl at 25 ± 1 °C, as measured by impedance, and polarization. It has been discovered that when the concentration of extract increases, the effectiveness of inhibition increases. Inhibition efficiency significantly increases as the extract concentration increases from 0 up to 200, 75, 400, and 100 ppm in acidic media. When the concentration of inhibitor exceeds from above values, the inhibition efficiency decreases slightly. The slight change in inhibition efficiency is due to the saturation adsorption of inhibitor molecules on the alloy surface. The higher inhibition efficiency indicates that the Dracocephalum extract is a suitable corrosion inhibitor for both acidic media.
Adsorption isotherm. Adsorption isotherms serve a critical function in providing extensive information about the current interaction behavior between metal surfaces, and Dracocephalum extract molecules 52 .
Different adsorption isotherm models were utilized in this work to suit the experimental results. The Langmuir isotherm is in good agreement with the experimental findings. The general form of the Langmuir isotherm model is shown in below equation [53][54][55][56][57] : www.nature.com/scientificreports/ where, θ , K ads , and C are the metal surface coverage, the equilibrium constant for the adsorption-desorption process, and the inhibitor concentration, respectively. As can be seen, when a graph is drawn between (C/θ), and C, a straight line (R 2 > 0.9) is formed for all samples, as shown in Fig. 8, with a gradient (slope) near to the unit and an intercept equal to K ads . The fact that all linear correlation coefficients (R) are almost equal to one shows that plant extract adsorption on mild steel surfaces follows the Langmuir adsorption isotherm. The Langmuir isotherm implies inhibitor molecule monolayer adsorption, or the inhibitor molecule occupies one active site on a metal surface 58 . Furthermore, the Langmuir adsorption isotherm revealed that organic components in plant extracts with polar atoms or groups adsorbed on the metal surface may interact via mutual attraction or repulsion 59 . The calculated adsorption coefficient, K ads , was larger in H 2 SO 4 than in HCl, indicating that the adsorption of inhibitor molecules on active sites of steel surfaces was easier in H 2 SO 4 than in HCl solution 60 . The strength, and stability of the adsorbed layer formed by nano extract in both solutions could also be evaluated from the higher K ads value compared to the other situation.
The standard adsorption free energy ( G o ads ) is also calculated using the K ads values. In the context of corrosion inhibition, physisorption, and chemisorption are two adsorption mechanisms that are frequently studied 61 . For the physical adsorption, values of the standard adsorption free energy are until − 20 kJ/mol, while those lower than − 40 kJ/mol are correlated with the chemical adsorption 3,53 .
G o ads of the adsorption process linked with K ads , and determined using below equation 62 : where, R, and T are the universal gas constant, and thermodynamic temperature, respectively, and 10 6 points to the ppm concentration (mg/L) of water. For H 2 SO 4 solution with bulk, and nano size of extract the calculated value of G o ads is 28.54, and − 31.71 kJ/ mol, respectively. For HCl solution with bulk, and nano size of extract, the calculated value of G o ads is 22.83, and − 29.70 kJ/mol, respectively. As a result of the obtained value for G o ads , it can be concluded that Dracocephalum adsorption is not solely chemisorption or physisorption, but also includes comprehensive adsorption (both chemical and physical), and that the negative sign of G o ads indicates that inhibitor molecule adsorption on the metal surface is spontaneous 3 . Table 5 lists the results, including K ads , and G o ads .
(3) C θ = 1 K ads + C   Table 6. It can be concluded that Dracocephalum extract in bulk, and especially in nanometer size is a suitable candidate for boosting the corrosion resistance of mild steel alloy in 0.5 M H 2 SO 4 , and 1.0 M HCl.
Mechanism of corrosion inhibition. The examined compounds' ability to prevent carbon steel corrosion is mostly owing to their physical or chemical adsorption on the metal surface, where they replace H 2 O molecules on the steel surface, and form a compact barrier coating 71 . Electrostatic contact occurs between charged inhibitor molecules, and charged metal surfaces in the event of physical adsorption (Fig. 9a). During chemical adsorption, the pair electron on the π-electron of multiple bonds, and heteroatoms interact with the iron's unoccupied d-orbitals (Fig. 9b) 13 . In this work, the values of G o ads are − 22.83, and 29.70 kJ mol −1 , in HCl solution, indicating that the examined compound molecules are adsorbed by a mix of chemical, and physical adsorption. It is known experimentally that the steel surface is positively charged in acidic solutions, Cl − ions may be adsorbed on the positively charged steel surface, and subsequently, the protonated inhibitor molecules are adsorbed via electrostatic attraction (physical adsorption). But at the same time, d-orbitals of iron atoms get a lone pair of electrons on π-electron, and heteroatoms in the extract structure (Chemical adsorption). In the H 2 SO 4 solution,

Conclusions
The effect of Dracocephalum extract based on bulk, and nanometer size as a corrosion inhibitor for mild steel in 0.5 M H 2 SO 4 , and 1.0 M HCl solutions was investigated: 1. The data derived from EIS, and PP curves indicate that the inhibition efficiency augmented with the increase in extract concentration up to a special dose. 2. By polarization method, in HCl solution, the highest IE% is 88% at the best dose of nano extract (100 ppm), but the highest IE% is 90% at best dose of the bulk extract (400 ppm). In the H 2 SO 4 solution, the highest IE% is 89% at the best dose of the bulk extract (200 ppm), but the corrosion inhibitor had the best inhibition  www.nature.com/scientificreports/ efficiency (94%), at the minimum concentration (75 ppm) of nano extract. It was worth noting that the value of IE% calculated by PP shows the same trend as that obtained from the EIS curves method. 3. In both acidic environments, PP measurements reveal that this examined chemical reduced corrosion by mixed-type inhibition, impacting both hydrogen evolution, and metal dissolution, with a predominant anodic action in the H 2 SO 4 medium. 4. According to EIS, this compound reduced corrosion through adsorption on the metal/solution contact.

5.
G o ads suggested that Dracocephalum adsorption is not only chemisorption or physisorption but also includes comprehensive adsorption. That means, the investigated compound adsorbed both chemical, and physical adsorption on the st-37 surface while following the Langmuir isotherm. Furthermore, the negative value of G o ads shows that inhibitor molecules adsorb spontaneously on the metal surface. www.nature.com/scientificreports/ 6. Optical, and SEM microscopy were used to confirm the corrosion testing. Thus, a uniform and less damaged surface was found with the optimal concentration of Dracocephalum extract in both acid solutions. The corrosion inhibition effectiveness showed up, and the protective inhibitor film was formed.
Finally, compared to the results of other researchers, it can be concluded that the Dracocephalum extract has the lowest optimal concentration, and proper efficiency. Therefore, by using Dracocephalum extract based on nanometer size, we could reduce the optimal concentration of inhibitor significantly, and increase the corrosion resistant, as well as efficiency. That is a cheap, eco-friendly, and efficient method to reduce the corrosion of mild steel in acidic media. So, Dracocephalum extract can be a suitable candidate for boosting the corrosion resistance of mild steel alloy in 0.5 M H 2 SO 4 , and 1.0 M HCl.

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.